The present invention relates to a color picture tube having an improved inline electron gun, and particularly to an improvement in the gun for obtaining equal raster sizes (also called coma correction) within the tube.
An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and direct those beams along convergent paths to a point or small area of convergence near the tube screen.
A problem that exists in a color picture tube having an inline gun is coma distortion wherein the sizes of the electron beam rasters scanned on the screen by an external magnetic deflection yoke are different because of the eccentricity of the two outer beams with respect to the center of the yoke. Messineo et al., U.S. Pat. No. 3,164,737, issued Jan. 5, 1965, teaches that a similar coma distortion caused by using different beam velocities can be corrected by use of a magnetic shield around the path of one or more beams in a three gun assembly. Barkow, U.S. Pat. No. 3,196,305, issued July 20, 1965, teaches the use of magnetic enhancers adjacent to the path of one or more beams in a delta gun, for the same purpose. Krackhardt et al., U.S. Pat. No. 3,534,208, issued Oct. 13, 1970, teaches the use of a magnetic shield around the middle one of three inline beams, for coma correction. Yoshida et al., U.S. Pat. No. 3,548,249, issued Dec. 15, 1970, teaches the use of C-shaped elements positioned between the center and outer beams to enhance the effect of the vertical deflection field on the center beam. Murata et al., U.S. Pat. No. 3,594,600, issued July 20, 1971, teaches the use of C-shaped shields around the outer beams, with the open sides of the members facing each other. These shields appear to shunt the vertical deflection field around all three beams. Takenaka et al., U.S. Pat. No. 3,860,850, issued Jan. 14, 1975, teaches the use of V-shaped enhancement members located above and below three inline beams and C-shaped shields around the two outer beams. Hughes, U.S. Pat. No. 3,873,879, issued March 25, 1975, teaches the use of small disc-shaped enhancement elements above and below the center beam and ring shaped shunts around the two outer beams.
All of the foregoing patents solved various raster size problems. More recently, U.S. Pat. No. 4,396,862, issued to Hughes on Aug. 2, 1983, discloses correction members that weaken the effect of the horizontal magnetic deflection field on the center beam and weaken the effect of both horizontal and vertical deflection fields on both of the outer beams. Such coma correction members have worked well on inline electron guns made to recent date. However, newer inline electron guns, such as disclosed in U.S. Pat. No. 4,370,592, issued to Hughes et al. on Jan. 25, 1983, and in U.S. Pat. No. 4,388,552, issued to Greninger on June 14, 1983, have coma correction problems which are similar but of a much lower magnitude. Although these problems can be solved by use of the coma correction members described in the Hughes U.S. Pat. No. 4,396,862, such members must be made so thin that they are very difficult to handle and become distorted when welded. Therefore, there is a need for a new coma correction member design, which will provide the more subtle lower magnitude coma correction required in the aforementioned newer electron guns, with the use of material having adequate thickness for ease of handling and which will not distort when welded. The present invention fulfills this need for a new coma correction member design.